Ferritic stainless steel excellent in heat resistance property and formability

ABSTRACT

An object is to provide ferritic stainless steel excellent in heat resistance (oxidation resistance, a thermal fatigue property and a high-temperature fatigue property) and formability, while preventing a decrease in oxidation resistance due to Cu, without adding expensive chemical elements such as Mo and W. Specifically, ferritic stainless steel having a chemical composition containing, by mass %, C: 0.015% or less, Si: 0.4% or more and 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.010% or less, Cr: 12% or more and less than 16%, N: 0.015% or less, Nb: 0.3% or more and 0.65% or less, Ti: 0.15% or less, Mo: 0.1% or less, W: 0.1% or less, Cu: 1.0% or more and 2.5% or less and Al: 0.2% or more and 1.0% or less, while the relationship Si≧Al is satisfied, and the balance being Fe and inevitable impurities.

TECHNICAL FIELD

The present invention relates to ferritic stainless steel having highheat resistance (a thermal fatigue property, oxidation resistance and ahigh-temperature fatigue property) and formability which can be ideallyused for the parts of an exhaust system which are used in a hightemperature environment such as an exhaust pipe and a catalyst outercylinder (also called a converter case) of an automobile and amotorcycle and an exhaust air duct of a thermal electric power plant.

BACKGROUND ART

The parts of an exhaust system such as an exhaust manifold, an exhaustpipe, a converter case and a muffler which are used in the environmentof the exhaust system of an automobile are required to be excellent in athermal fatigue property, a high-temperature fatigue property andoxidation resistance (hereinafter, these properties are collectivelycalled heat resistance). Since the parts such as an exhaust manifold aresubjected to heating and cooling due to the repetition of start and stopof engine operation in a state in which they are restrained by thesurrounding parts, the thermal expansion and contraction of the materialof the parts are restricted, which results in the occurrence of thermalstrain. The fatigue phenomenon due to this thermal strain is thermalfatigue. On the other hand, the parts are continuously subjected tovibration while they are heated in the initiation of engine operation.The fatigue phenomenon due to the accumulation of strain caused by thisvibration is high-temperature fatigue. The former is low-cycle fatigueand the latter is high-cycle fatigue and both are completely differentfatigue phenomena.

For applications in which heat resistance are required as describedabove, nowadays, Cr containing steel to which Nb and Si are added suchas Type429 (containing 14Cr-0.9Si-0.4Nb) is often used. However, sincean exhaust gas temperature has become higher than 900° C. with theimprovement of engine performance, the thermal fatigue property ofType429 has become unsatisfactory.

In order to solve this problem, Cr containing steel having ahigh-temperature yield strength increased by adding Nb and Mo, SUS444(containing 19Cr-0.5Nb-2Mo) conforming to JIS G 4305 and ferriticstainless steel containing less Cr to which Nb, Mo and W are added andthe like have been developed (refer to, for example, Patent Literature1). However, since the prices of rare metals such as Mo and W have beenmarkedly rising recently, the development of a material having heatresistance equivalent to those of these kinds of steel by usinginexpensive raw materials has become to be required.

Examples of materials having excellent heat resistance without usingexpensive chemical elements such as Mo and W are disclosed by PatentLiteratures 2 through 4. Patent Literature 2 discloses ferriticstainless steel to be used for the parts of an exhaust gas flow channelof an automobile. In Patent Literatures 2, Nb: 0.50 mass % or less, Cu:0.8 mass % or more and 2.0 mass % or less and V: 0.03 mass % or more and0.20 mass % or less are added to steel having a Cr content of 10 mass %or more and 20 mass % or less. Patent Literature 3 discloses ferriticstainless steel excellent in a thermal fatigue property. In PatentLiteratures 3, Ti: 0.05 mass % or more and 0.30 mass % or less, Nb: 0.10mass % or more and 0.60 mass % or less, Cu: 0.8 mass % or more and 2.0mass % or less and B: 0.0005 mass % or more and 0.02 mass % or less areadded to steel having a Cr content of 10 mass % or more and 20 mass % orless. Patent Literature 4 discloses ferritic stainless steel to be usedfor the parts of an exhaust gas flow channel of an automobile. In PatentLiteratures 4, Cu: 1 mass % or more and 3 mass % or less is added tosteel having a Cr content of 15 mass % or more and 25 mass % or less.These kinds of disclosed steel are all characterized by having a thermalfatigue property improved by adding Cu.

CITATION LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Application Publication No.    2004-018921-   [PTL 2] International Publication No. WO2003/004714-   [PTL 3] Japanese Unexamined Patent Application Publication No.    2006-117985-   [PTL 4] Japanese Unexamined Patent Application Publication No.    2000-297355

SUMMARY OF INVENTION Technical Problem

However, according to investigations carried out by the presentinventors, in the case where Cu is added as in the methods disclosed byPatent Literatures 2 through 4, it has been found that, while a thermalfatigue property is improved, contrarily oxidation resistance isdecreased, which results in the deterioration of the overall heatresistance.

In addition, since a space which an exhaust manifold can occupy in anengine space has become smaller with the weight reduction of anautomobile, it has come to be required that an exhaust manifold can beformed into a complex shape.

The present invention has been completed in view of the situationdescribed above, and an object of the present invention is to provideferritic stainless steel excellent in heat resistance (oxidationresistance, a thermal fatigue property and a high-temperature fatigueproperty) and formability, while preventing a decrease in oxidationresistance due to Cu, without adding expensive chemical elements such asMo and W.

Incidentally, the meaning of “excellent in heat resistance” according tothe present invention is that oxidation resistance, a thermal fatigueproperty and a high-temperature fatigue property are equivalent to orbetter than those of SUS444. Specifically, it means that oxidationresistance at a temperature of 950° C. is equivalent to or better thanthat of SUS444, that a thermal fatigue property when temperaturefluctuations repeatedly occur between the temperatures of 100° C. and850° C. is equivalent to or better than that of SUS444 and that ahigh-temperature fatigue property at a temperature of 850° C. isequivalent to or better than that of SUS444. In addition, the meaning of“excellent in formability” according to the present invention is that amean elongation in the three directions at room temperature is 36% ormore.

Solution to Problem

The present inventors diligently conducted investigations in order todevelop ferritic stainless steel excellent in oxidation resistance and athermal fatigue property by preventing a decrease in oxidationresistance due to Cu which occurs in the conventional methods withoutadding expensive chemical elements such as Mo or W. As a result, thepresent inventors found that a high strength in high-temperature can beachieved by adding the combination of Nb: 0.3 mass % or more and 0.65mass % or less and Cu: 1.0 mass % or more and 2.5 mass % or less. Bygetting the high strength, a thermal fatigue property can be improved ina wide temperature range. The present inventors found that a decrease inoxidation resistance due to the addition of Cu can be prevented byadding an appropriate amount of Al (0.2 mass % or more and 1.0 mass % orless). The present inventors found that, therefore, heat resistance (athermal fatigue property and oxidation resistance) equivalent to orbetter than that of SUS444 can be achieved only by controlling thecontents of Nb, Cu and Al to the appropriate range as described abovewithout adding Mo or W. In addition, the present inventors diligentlyconducted investigations regarding a method for improving oxidationresistance in an environment containing water vapor which is assumed inthe case where the ferritic stainless steel is practically used for anexhaust manifold and the like, and found that oxidation resistance in anatmosphere containing water vapor (hereinafter, called water vaporoxidation resistance) also becomes equivalent to or better than that ofSUS444 by adjusting a Si content (0.4 mass % or more and 1.0 mass % orless).

In addition, a fatigue resistance property against vibration inpractical service conditions of the parts of the exhaust system of anautomobile such as an exhaust manifold is also important. Therefore, thepresent inventors diligently conducted investigations regarding a methodfor improving a high-temperature fatigue property, and found that ahigh-temperature fatigue property also becomes equivalent to or betterthan that of SUS444 by adjusting the balance of a Si content and an Alcontent (Si≧Al).

Moreover, the present inventors diligently conducted investigationsregarding the influence of Cr on formability and oxidation resistance,and found that formability can be improved by reducing a Cr contentwithout there being a significant influence on oxidation resistance.

Although it has been well known in the past that formability can beimproved by reducing a Cr content. But there is a decrease in oxidationresistance by reducing a Cr content. The decrease in oxidationresistance has been compensated for by adding Mo and W, instead of Cr,in the past as disclosed by Patent Literature 1. In contrast to this,according to the present invention, it has been found that bothexcellent oxidation resistance and formability can be achieved by addingan appropriate amount of Al without adding expensive chemical elementssuch as Mo and W, even if a Cr content is reduced.

The present invention has been completed on the basis of the knowledgeof the present inventors described above.

That is to say, the present invention provides ferritic stainless steelexcellent in heat resistance and formability having a chemicalcomposition containing, by mass %, C, 0.015% or less, Si: 0.4% or moreand 1.0% or less, Mn: 1.0% or less, P: 0.040% or less, S: 0.010% orless, Cr: 12% or more and less than 16%, N: 0.015% or less, Nb: 0.3% ormore and 0.65% or less, Ti: 0.15% or less, Mo: 0.1% or less, W: 0.1% orless, Cu: 1.0% or more and 2.5% or less and Al: 0.2% or more and 1.0% orless, while the relationship Si≧Al is satisfied, and the balance beingFe and inevitable impurities.

In addition, the present invention provides ferritic stainless steelexcellent in heat resistance and formability having a chemicalcomposition further containing one, two or more chemical elementsselected from among, by mass %, B: 0.003% or less, REM: 0.08% or less,Zr: 0.5% or less, V: 0.5% or less, Co: 0.5% or less and Ni: 0.5% orless.

Advantageous Effects of Invention

According to the present invention, ferritic stainless steel having heatresistance (a thermal fatigue property, oxidation resistance and ahigh-temperature fatigue property) equivalent to or better than that ofSUS444 (JIS G 4305) and excellent formability can be obtained withoutadding expensive Mo or W. Therefore, the steel according to the presentinvention can be ideally used for the parts of the exhaust system of anautomobile.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a thermal fatigue test specimen.

FIG. 2 is a diagram illustrating conditions of temperature andconstraint in a thermal fatigue test.

FIG. 3 is a diagram illustrating a high-temperature fatigue testspecimen.

FIG. 4 is a graph illustrating the influence of a Cu content on athermal fatigue property.

FIG. 5 is a graph illustrating the influence of an Al content onoxidation resistance (an increase in weight due to oxidation).

FIG. 6 is a graph illustrating the influence of a Si content on watervapor oxidation resistance (an increase in weight due to oxidation).

FIG. 7 is a graph illustrating the influence of a Si content—an Alcontent (Si—Al) on a high-temperature fatigue property.

FIG. 8 is a graph illustrating the influence of a Cr content on watervapor oxidation resistance (an increase in weight due to oxidation).

FIG. 9 is a graph illustrating the influence of a Cr content on a meanelongation in the three directions at room temperature.

DESCRIPTION OF EMBODIMENTS

Firstly, the fundamental experiments which led to the completion of thepresent invention will be described. Hereinafter, % used when describingchemical composition always denotes mass %.

Steel having a basic chemical composition containing C, 0.005% or moreand 0.007% or less, N: 0.004% or more and 0.006% or less, P: 0.02% ormore and 0.03% or less, S: 0.002% or more and 0.004% or less, Si: 0.85%,Mn: 0.4%, Cr: 14%, Nb: 0.45%, Al: 0.35%, Ti: 0.007%, Mo: 0.01% or moreand 0.03% or less and W: 0.01% or more and 0.03% or less and a Cucontent which was adjusted variously in the range of 0% or more and 3%or less was smelted by using an experimental method and made into asteel ingot of 50 kg, then the steel ingot was subjected to forging anda heat treatment into a steel material having a cross section of 35mm×35 mm, then a thermal fatigue test specimen having the dimensionsillustrated in FIG. 1 was made of the steel material. Then, the thermalfatigue life of the specimen was observed by performing a thermal cycleheat treatment in which a restraint ratio was 0.30 and in which heatingand cooling were repeated so that temperature fluctuations repeatedlyoccurred between 100° C. and 850° C. as illustrated in FIG. 2. Thethermal fatigue life represents as the number of cycles at which thestress first started to continuously decrease from that in the previouscycle. The stress was derived by calculated as the quotient of the loaddetected at 100° C. divided by the cross section area of the soakedparallel portion of a test specimen indicated in FIG. 1. This number ofcycles corresponded to that at which a crack occurred in the testspecimen. Incidentally, a similar test was performed with SUS444 (19%Cr-2% Mo-0.5% Nb steel) for comparison.

FIG. 4 illustrates the influence of Cu content on thermal fatigue lifein the thermal fatigue test described above. This figure indicates thatthermal fatigue life equivalent to or longer than that of SUS444 (about1350 cycles) can be achieved by setting the Cu content to be 1.0% ormore. Therefore, it is necessary that the Cu content be 1.0% or more inorder to improve a thermal fatigue property.

Steel having a basic chemical composition containing C, 0.006%, N:0.007%, P: 0.02% or more and 0.03% or less, S: 0.002% or more and 0.004%or less, Mn: 0.2%, Si: 0.85%, Cr: 14%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%,Mo: 0.01% or more and 0.03% or less and W: 0.01% or more and 0.03% orless and an Al content which was adjusted variously in the range of 0%or more and 2% or less was smelted by using an experimental method andmade into a steel ingot of 50 kg, then the steel ingot was subjected tohot rolling, hot rolled annealing, cold rolling and finishing annealingand made into a cold rolled and annealed steel sheet having a thicknessof 2 mm. A test specimen of 30 mm×20 mm was cut out of the cold rolledsteel sheet obtained as described above, then a hole of 4 mmφ waspunched in the upper part of the test specimen, then the surface and theedge face of the specimen was polished with a #320 emery paper, thendegreased and then used in a continuous oxidation test in air describedbelow.

<Continuous Oxidation Test in Air>

The test specimen described above was held in a furnace in air at atemperature of 950° C. for a duration of 200 hours, and then an increasein weight per unit area due to oxidation (g/m²) was derived from theobserved difference in the mass of the test specimen before and afterthe heating test.

FIG. 5 illustrates the influence of Al content on the increase in weightdue to oxidation in the continuous oxidation test in air describedabove. This figure indicates that an oxidation resistance equivalent toor better than that of SUS444 (increase in weight due to oxidation: 19g/m² or less) can be achieved by setting the Al content to be 0.2% ormore.

Steel having a basic chemical composition containing C, 0.006%, N:0.007%, P: 0.02% or more and 0.03% or less, S: 0.002% or more and 0.004%or less, Mn: 0.2%, Al: 0.45%, Cr: 14%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%,Mo: 0.01% or more and 0.03% or less and W: 0.01% or more and 0.03% orless and a Si content which was adjusted variously was smelted by usingan experimental method and made into a steel ingot of 50 kg. Then thesteel ingot was subjected to hot rolling, hot rolled annealing, coldrolling and finishing annealing and made into a cold rolled and annealedsteel sheet having a thickness of 2 mm. A test specimen of 30 mm×20 mmwas cut out of the cold rolled steel sheet obtained as described above.Then a hole of 4 mmφ was punched in the upper part of the test specimen,then the surface and the edge face of the specimen was polished with a#320 emery paper. Then degreased and then used in a continuous oxidationtest in water vapor atmosphere described below.

<Continuous Oxidation Test in Water Vapor Atmosphere>

The test specimen described above was held in a furnace in a water vaporatmosphere in which a gas of 10 vol % CO₂-20 vol % H₂O-5 vol % O₂-bal.N₂ was blown at a rate of 0.5 L/min, and then an increase in weight perunit area due to oxidation (g/m²) was derived from the observeddifference in the mass of the specimen before and after the heatingtest.

FIG. 6 illustrates the influence of the Si content on the increase inweight due to oxidation in the oxidation test in water vapor atmospheredescribed above. This figure indicates that water vapor oxidationresistance equivalent to that of SUS444 (increase in weight due tooxidation: 37 g/m² or less) cannot be achieved, unless the Si content isset to be 0.4% or more.

Steel having a basic chemical composition containing C, 0.006%, N:0.007%, P: 0.02% or more and 0.03% or less, S: 0.002% or more and 0.004%or less, Mn: 0.2%, Cr: 14%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%, Mo: 0.01%or more and 0.03% or less and W: 0.01% or more and 0.03% or less and thecontents of Si and Al which were adjusted variously was smelted by usingan experimental method and made into a steel ingot of 50 kg. Then thesteel ingot was subjected to hot rolling, hot rolled annealing, coldrolling and finishing annealing and made into a cold rolled and annealedsteel sheet having a thickness of 2 mm. A high-temperature fatigue testspecimen having a shape illustrated in FIG. 3 was made of the coldrolled steel sheet obtained as described above and then used in ahigh-temperature fatigue test described below.

<High-Temperature Fatigue Test>

The high-temperature fatigue property of the test specimen describedabove was evaluated by using a Schenck type fatigue testing machine andby performing reversed vibration of 22 Hz (1300 rpm) at a temperature of850° C. Here, a bending stress of 70 MPa was exerted on the surface ofthe steel sheet during the test, and the fatigue property was evaluatedin terms of a number of cycles until fracture occurred.

FIG. 7 illustrates the influence of Si—Al on the number of cycles in thehigh-temperature fatigue test described above. This figure indicatesthat it is necessary to satisfy the relationship Si≧Al in order toachieve a high-temperature fatigue property equivalent to that of SUS444(24×10⁵ cycles).

Steel having a basic chemical composition containing C, 0.006%, N:0.007%, P: 0.02% or more and 0.03% or less, S: 0.002% or more and 0.004%or less, Mn: 0.2% Si: 0.85%, Al: 0.45%, Nb: 0.49%, Cu: 1.5%, Ti: 0.007%,Mo: 0.01% or more and 0.03% or less and W: 0.01% or more and 0.03% orless and a Cr content which was adjusted variously was smelted by usingan experimental method and made into a steel ingot of 50 kg. Then thesteel ingot was subjected to hot rolling, hot rolled annealing, coldrolling and finishing annealing and made into a cold rolled and annealedsteel sheet having a thickness of 2 mm. A test specimen of 30 mm×20 mmwas cut out from the cold rolled steel sheet obtained as describedabove, then a hole of 4 mmφ was punched in the upper part of the testspecimen. Then the surface and the edge face of the specimen waspolished with a #320 emery paper, then degreased and then used in theoxidation test in water vapor atmosphere described above.

FIG. 8 illustrates the influence of the Cr content on the increase inweight due to oxidation in the oxidation test in water vapor atmospheredescribed above. This figure indicates that water vapor oxidationresistance equivalent to that of SUS444 (increase in weight due tooxidation: 37 g/m² or less) can be achieved in the case where the Crcontent is 12% or more.

In addition, tensile tests were conducted at room temperature withtensile test pieces conforming to JIS NO. 13B which were made of thesecold rolled and annealed steel sheets. Tensile test pieces had thedirections of tension respectively in the rolling direction (Ldirection), in the direction at right angles to the rolling direction (Cdirection) and in the direction at a 45° angles to the rolling direction(D direction). A mean elongation was derived from the breakingelongations which were obtained by performing tensile tests in the threedirections at room temperature and calculated by the equation below.

Mean elongation El(%)=(E _(L)+2E _(D) +E _(C))/4,

where E_(L): El (%) in L direction, E_(D): El (%) in D direction andE_(C): El (%) in C direction.

FIG. 9 illustrates the influence of the Cr content on the mean value ofelongations in the three directions (L, C and D directions) in thetensile test. This figure indicates that excellent formability in termsof the mean elongation in the three directions (L, C and D directions)of 36% or more can be achieved in the case where the Cr content is lessthan 16%.

The present invention has been completed by conducting furtherinvestigations on the basis of the results of the fundamentalexperiments described above.

The ferritic stainless steel according to the present invention will bedescribed in detail hereafter.

Firstly, the chemical composition according to the present inventionwill be described.

C: 0.015% or Less

Although C is a chemical element which is effective for increasing thestrength of steel, there is a significant decrease in toughness andformability in the case where C content is more than 0.015%. Therefore,according to the present invention, the C content is set to be 0.015% orless. Incidentally, it is preferable that the C content be as small aspossible from the viewpoint of achieving formability and that the carboncontent be 0.008% or less. On the other hand, it is preferable that theC content be 0.001% or more in order to achieve strength which isrequired of the parts of an exhaust system, more preferably 0.002% ormore and 0.008% or less.

Si: 0.4% or More and 1.0% or Less

Si is a chemical element which is important for improving oxidationresistance in a water vapor atmosphere. As FIG. 6 indicates, it isnecessary to set Si content to be 0.4% or more in order to achieve watervapor oxidation resistance equivalent to that of SUS444. On the otherhand, there is a significant decrease in formability in the case wherethe Si content is more than 1.0%. Therefore, the Si content is set to be0.4% or more and 1.0% or less, preferably 0.5% or more and 0.9% or less.Although the mechanism through which water vapor oxidation resistance isincreased in the case where the Si content is 0.4% or more is not clear,it is considered that a dense continuous oxide layer is formed on thesurface of a steel sheet in the case where the Si content is 0.4% ormore and the penetration of gas elements from outside is prevented,which results in an increase in water vapor oxidation resistance. It ispreferable that the Si content be 0.5% or more in the case where thereis a necessity of oxidation resistance in a severer environment.

Mn: 1.0% or Less

Although Mn is a chemical element which causes an increase in thestrength of steel and which is effective as a deoxidation agent, a γphase tends to be formed at a high temperature in the case where Mncontent is excessively large, the excessive Mn content results in adecrease in heat resistance. Therefore, the Mn content is set to be 1.0%or less, preferably 0.7% or less. It is preferable that the Mn contentbe 0.05% or more in order to realize the effect of increasing strengthand deoxidation.

P: 0.040% or Less

Since P is a harmful chemical element which causes a decrease inductility, it is preferable that P content be as small as possible.Therefore, the P content is set to be 0.040% or less, preferably 0.030%or less.

S: 0.010% or Less

Since S is a harmful chemical element which causes a decrease inelongation and an r value. S has a negative influence on formability andS causes a decrease in corrosion resistance which is the basic propertyof stainless steel. It is preferable that S content be as small aspossible. Therefore, the S content is set to be 0.010% or less,preferably 0.005% or less.

Cr: 12% or More and Less than 16%

Cr is a chemical element which is effective for increasing corrosionresistance and oxidation resistance, which are the characteristics ofstainless steel. Sufficient oxidation resistance cannot be achieved inthe case where Cr content is less than 12%. On the other hand, Cr is achemical element which causes an increase in hardness and a decrease inductility of steel at room temperature by solid solution strengthening,in particular, these negative influences become significant in the casewhere the Cr content is 16% or more. Therefore, the Cr content is set tobe 12% or more and less than 16%, preferably 12% or more and 15% orless.

N: 0.015% or Less

N is a chemical element which causes a decrease in the ductility and theformability of steel, and these negative influences are significant inthe case where N content is more than 0.015%. Therefore, the N contentis set to be 0.015% or less. Incidentally, it is preferable that the Ncontent be as small as possible from the viewpoint of achievingductility and formability and that the N content be less than 0.010%.

Nb: 0.3% or More and 0.65% or Less

Nb is a chemical element which is effective for increasing corrosionresistance, formability and intergranular corrosion resistance at weldsby forming carbide, nitride and carbonitride in combination with C andN. Nb is effective for increasing a thermal fatigue property byincreasing high-temperature strength. These effects can be realized bysetting Nb content to be 0.3% or more. On the other hand, a Laves phase(Fe₂Nb) tends to be precipitated in the case where the Nb content ismore than 0.65%, which results in the acceleration of embrittlement.Therefore, the Nb content is set to be 0.3% or more and 0.65% or less,preferably 0.4% or more and 0.55% or less.

Mo: 0.1% or Less

Since Mo is an expensive chemical element, additionally in view of thepurpose of the present invention, Mo is not added positively. However,Mo may be mixed in from the material of steel such as scrap in the rangeof 0.1% or less. Therefore, Mo content is set to be 0.1% or less.

W: 0.1% or Less

Since W is an expensive chemical element like Mo, additionally in viewof the purpose of the present invention, W is not added positively.However, W may be mixed in from the material of steel such as scrap inthe range of 0.1% or less. Therefore, W content is set to be 0.1% orless.

Cu: 1.0% or More and 2.5% or Less

Cu is a chemical element which is very effective for improving a thermalfatigue property. As FIG. 3 indicates, it is necessary that Cu contentbe 1.0% or more in order to achieve a thermal fatigue propertyequivalent to or better than that of SUS444. However, in the case wherethe Cu content is more than 2.5%, ε-Cu is precipitated when cooling isperformed after a heat treatment, which results in an increase in thehardness of steel and results in embrittlement tending to occur when hotwork is performed. More importantly, while a thermal fatigue property isimproved by adding Cu, contrarily the oxidation resistance of steel isdecreased, which results in the deterioration of the overall heatresistance. The reason for this has not been fully identified. However,Cu seems to concentrate in a Cr depletion layer where scale has formedthereon and prevent Cr, an element that should improve intrinsicoxidation resistance of stainless steel, from diffusing again.Therefore, the Cu content is set to be 1.0% or more and 2.5% or less,preferably 1.1% or more and 1.8% or less.

Ti: 0.15% or Less

Ti is effective for improving corrosion resistance, formability andintergranular corrosion resistance of a welded part by fixing C and Nlike Nb does. However, this effect saturates and there is an increase inthe hardness of steel in the case where Ti content is more than 0.15% inthe present invention in which Nb is contained. Therefore, the Ticontent is set to be 0.15% or less. Since Ti has higher affinity for Nthan Nb does, Ti tends to form TiN of a large size. Since TIN of a largesize tends to become the origin of a crack and causes a decrease intoughness, it is preferable that the Ti content be 0.01% or less in thecase where the toughness of a hot rolled steel sheet is necessary.Incidentally, since it is not necessary to positively add Ti in thepresent invention, the lower limit of the Ti content includes 0%.

Al: 0.2% or More and 1.0% or Less

Al is a chemical element which is essential for increasing the oxidationresistance of Cu containing steel as FIG. 5 indicates. In addition,since Al is effective as a solid solution strengthening element and, inparticular, is effective for increasing high-temperature strength at atemperature of higher than 800° C., Al is a chemical element which isimportant for improving a high-temperature fatigue property in thepresent invention. It is necessary that Al content be 0.2% or more inorder to achieve oxidation resistance equivalent to or better than thatof SUS444. On the other hand, there is a decrease in formability due toan increase in the hardness of steel in the case where the Al content ismore than 1.0%. Therefore, the Al content is set to be 0.2% or more and1.0% or less, preferably 0.3% or more and 1.0% or less, more preferably0.3% or more and 0.5% or less.

Si≧Al

Since Al is effective as a solid solution strengthening element and, inparticular, effective for increasing high-temperature strength at a hightemperature of higher than 800° C., Al is a chemical element which isimportant for improving a high-temperature fatigue property in thepresent invention as described above, and Si is a chemical element whichis important for effectively utilizing this effect of solid solutionstrengthening of Al. In the case where the amount of Si is less thanthat of Al, there is a decrease in the amounts of solid solution Al,because Al preferentially forms oxides and nitrides at high temperature,which decreases the contribution of Al to strengthening. On the otherhand, in the case where the amount of Si is larger than that of Al, Siis preferentially oxidized and forms a dense continuous oxide layer onthe surface of a steel sheet. Since this oxide becomes a barrier to thediffusion of oxygen and nitrogen, Al is kept in the state of a solidsolution without being oxidized or nitrided, which makes it possible toimprove a high-temperature fatigue property by strengthening steelthrough solid solution strengthening. Therefore, it is necessary thatthe relationship Si≧Al be satisfied in order to achieve ahigh-temperature fatigue property equivalent to or better than that ofSUS444.

One, two or more chemical elements selected from among B, REM, Zr, V, Coand Ni may be further contained in the ferritic stainless steelaccording to the present invention in addition to the chemicalcomposition described above.

B: 0.003% or Less

B is a chemical element which is effective for improving formability, inparticular, secondary formability. However, in the case where B contentis more than 0.003%, B causes a decrease in formability by forming BN.Therefore, in the case where B is contained, the B content is set to be0.003% or less. Since the effect described above is realized in the casewhere the B content is 0.0004% or more, it is more preferable that the Bcontent be 0.0004% or more and 0.003% or less.

REM: 0.08% or Less and Zr: 0.5% or Less

REM (rare earth elements) and Zr are chemical elements which areeffective for improving oxidation resistance and may be added as neededin the present invention. However, in the case where the content of REMis more than 0.080%, the steel become easier to occur the embrittlecrack and, in the case where Zr content is more than 0.50%, the steelalso become easier to occur the embrittle crack due to the precipitationof a Zr intermetallic compound. Therefore, in the case where REM iscontained, the content of REM is set to be 0.080% or less, and, in thecase where Zr in contained, the Zr content is set to be 0.50% or less.Since the effect described above is realized in the case where thecontent of REM is 0.01% or more and in the case where the Zr content is0.0050% or more, it is preferable that the content of REM be 0.001% ormore and 0.080% or less and that the Zr content be 0.0050% or more and0.50% or less.

V: 0.5% or Less

V is a chemical element which is effective for improving formability andoxidation resistance. However, in the case where V content is more than0.50%, V(C,N) of a large size is precipitated, which results in thedeterioration of surface quality. Therefore, in the case where V iscontained, the V content is set to be 0.50% or less. It is preferablethat the V content be 0.15% or more and 0.50% or less in order torealize the effect of improving formability and oxidation resistance,more preferably 0.15% or more and 0.4% or less.

Co: 0.5% or Less

Co is a chemical element which is effective for improving toughness.However, Co is an expensive chemical element and the effect of Cosaturates in the case where Co content is more than 0.5%. Therefore, inthe case where Co is contained, the Co content is set to be 0.5% orless. Since the effect described above is effectively realized in thecase where the Co content is 0.02% or more, it is preferable that the Cocontent be 0.02% or more and 0.5% or less, more preferably 0.02% or moreand 0.2% or less.

Ni: 0.5% or Less

Ni is a chemical element which improves toughness. However, since Ni isexpensive and a chemical element which strongly forms a γ phase, Nicauses a decrease in oxidation resistance by forming a γ phase at a hightemperature in the case where Ni content is more than 0.5%. Therefore,in the case where Ni is contained, the Ni content is set to be 0.5% orless. Since the effect described above is effectively realized in thecase where the Ni content is 0.05% or more, it is preferable that the Nicontent be 0.05% or more and 0.5% or less, more preferably 0.05% or moreand 0.4% or less.

The remainder of the chemical composition consists of Fe and inevitableimpurities. Among the inevitable impurities, it is preferable that an Ocontent be 0.010% or less, a Sn content be 0.005% or less, a Mg contentbe 0.005% or less and a Ca content be 0.005% or less, more preferablythe 0 content be 0.005 or less, the Sn content be 0.003% or less, the Mgcontent be 0.003% or less and the Ca content be 0.003% or less.

The method for manufacturing the ferritic stainless steel will bedescribed hereafter.

The stainless steel according to the present invention may bemanufactured in a common method for manufacturing ferritic stainlesssteel and there is no particular limitation on manufacturing conditions.Examples of ideal manufacturing methods include smelting steel by usinga well-known, melting furnace such as a steel converter or an electricfurnace, further, optionally, making the steel have the chemicalcomposition according to the present invention described above byperforming secondary refining such as ladle refining or vacuum refining,then making a slab of the steel by using a continuous casting method oran ingot casting-blooming rolling method, and then making the slab acold rolled and annealed steel sheet through the processes such as hotrolling, hot rolled annealing, pickling, cold rolling, finishingannealing, pickling and so forth. Incidentally, the cold rollingdescribed above may be performed one time or repeated two times or morewith process annealing in between, and the processes of cold rolling,finishing annealing and pickling may be performed repeatedly. Moreover,optionally, hot rolled annealing may be omitted, and skin pass rollingmay be performed after cold rolling or finishing annealing in the casewhere brightness of a steel sheet is required.

Examples of more preferable manufacturing conditions are as follows.

It is preferable that some of the conditions of a hot rolling processand a cold rolling process be specified. In addition, in a steel makingprocess, it is preferable to smelt molten steel having the essentialchemical composition described above and the optional chemical elementsto be added as needed and to perform secondary refining by using a VODmethod (Vacuum Oxygen Decurbarization method). Although the smeltedmolten steel may be made into a steel material by using a well-knownmethod, it is preferable to use a continuous casting method from theviewpoint of productivity and material quality. The steel materialobtained through a continuous casting process is heated up to atemperature of, for example, from 1000° C. or higher and 1250° C. orlower, and then made into a hot rolled steel sheet having a specifiedthickness. It is needless to say that the steel material may be madeinto a material of a shape other than a sheet. This hot rolled steelsheet is subjected to, as needed, batch annealing at a temperature of600° C. or higher and 800° C. or lower or continuous annealing at atemperature of 900° C. or higher and 1100° C. or lower, and then madeinto a hot rolled sheet product after being descaled by performingpickling or the like. In addition, as needed, descaling may be performedby using a shot blasting method before pickling being performed.

Moreover, in order to obtain a cold rolled steel sheet, the hot rolledand annealed steel sheet obtained as described above is made into a coldrolled steel sheet through a cold rolling process. In this cold rollingprocess, in accordance with manufacturing circumstances, cold rollingmay be performed two times or more with process annealing in between asneeded. The total rolling ratio of the cold rolling process, in whichcold rolling is performed for one, two or more times, is set to be 60%or more, preferably 70% or more. The cold rolled steel sheet issubjected to continuous annealing (finishing annealing) at a temperatureof 900° C. or higher and 1150° C. or lower, preferably 950° C. or higherand 1120° C. or lower, and pickling, and then made into a cold rolledand annealed steel sheet. In addition, in accordance with useapplication, the shape of and the material quality of the steel sheetmay be adjusted by performing rolling with a light reduction ratio suchas skin pass rolling after cold rolled annealing being performed.

The hot rolled sheet product or cold rolled and annealed sheet productobtained as described above are formed into the exhaust pipe of anautomobile or a motor bicycle, a material to be used for a catalystouter cylinder, the exhaust air duct of a thermal electric power plant,or a part related to a fuel cell (such as a separator, an interconnector or a reformer) by performing bending work or other kinds ofwork in accordance with use application. There is no limitation onwelding methods for assembling these parts, and common arc weldingmethods such as MIG (Metal Inert Gas), MAG (Metal Active Gas) and TIG(Tungsten Inert Gas), resistance welding methods such as spot weldingand seam welding, high-frequency resistance welding methods such aselectric resistance welding and high-frequency induction welding methodsmay be applied.

EXAMPLES Example 1

Each of the steel No. 1 through 23 having chemical compositions given inTable 1 was smelted by using a vacuum melting furnace and made intosteel ingot of 50 kg, then the steel ingot was subjected to forging, andthen the forged ingot was divided into two pieces. Thereafter, one ofthe divided ingots was heated up to a temperature of 1170° C., thensubjected to hot rolling and made into a hot rolled steel sheet having athickness 5 mm, then subjected to hot rolled annealing, pickling, coldrolling with a rolling ratio of 60%, finishing annealing at atemperature of 1040° C., cooling at a cooling rate of 5° C./sec,pickling and then made into a cold rolled and annealed steel sheethaving a thickness of 2 mm. Each of the steel No. 1 through 11 is anexample in the range according to the present invention, and each of thesteel No. 12 through 23 is a comparative example out of the rangeaccording to the present invention. Incidentally, among the comparativeexamples, steel No. 19 has a chemical composition corresponding toType429, No. 20 has a chemical composition corresponding to SUS444, andNo. 21, No. 22 and No. 23 respectively have chemical compositionscorresponding to example 3 of Patent Literature 2, example 3 of PatentLiterature 3 and example 5 of Patent Literature 4.

The cold rolled steel sheets No. 1 through 23 were used in the two kindsof continuous oxidation tests, a high-temperature fatigue test and atensile test at room temperature as described below.

<Continuous Oxidation Test in Air>

A sample of 30 mm×20 mm was cut out of each of the cold rolled andannealed steel sheet obtained as described above, then a hole of 4 mmφwas punched in the upper part of the sample, then the surface and theedge face of the sample was polished with a #320 emery paper, thendegreased and then the sample was suspended in a furnace heated up to atemperature of 950° C. in air for a holding time of 200 hours. After thetest, the mass of the sample was observed, and then an increase inweight due to oxidation (g/m²) was calculated by deriving the differencebetween the mass observed before and after the test. Incidentally, thetest was repeated two times, and oxidation resistance in air wasevaluated by using the mean value of the difference in mass.

<Continuous Oxidation Test in Water Vapor Atmosphere>

A sample of 30 mm×20 mm was cut out from each of the cold rolled andannealed steel sheet obtained as described above. Then a hole of 4 mmφwas punched in the upper part of the sample, then the surface and theedge face of the sample was polished with a #320 emery paper and thendegreased. Thereafter, the sample was held in a furnace heated up to atemperature of 950° C. in a water vapor atmosphere in which a gas of 10vol % CO₂-20 vol % H₂O-5 vol % O₂-bal. N₂ was blown at a rate of 0.5L/min for a holding time of 200 hours, then, after the test, the mass ofthe sample was observed, and then an increase in weight due to oxidation(g/m²) was calculated by deriving the difference between the massobserved before and after the test.

<High-Temperature Fatigue Test>

A test specimen illustrated in FIG. 3 was cut out from the cold rolledand annealed steel sheet obtained as described above was subjected toreversed vibration of 1300 rpm (22 Hz) at a temperature of 850° C. byusing a Schenck type fatigue testing machine. Incidentally, a bendingstress of 70 MPa was exerted on the surface of the steel sheet duringthe test, and evaluation was done in terms of a number of cycles (cycle)until fracture occurred.

<Tensile Test at Room Temperature>

A tensile test piece conforming to JIS No. 13B which had the directionsof tension respectively in the rolling direction (L direction), in thedirection at right angle to the rolling direction (C direction) and inthe direction at 45° angles to the rolling direction (D direction) wascut out from the cold rolled and annealed steel sheet described above.Then tensile tests in these directions were conducted at roomtemperature, then breaking elongations were observed and then a meanelongation was derived by using the equation below.

Mean elongation El(%)=(E _(L)+2E _(D) +E _(C))/4,

where E_(L): El (%) in L direction, E_(D): El (%) in D direction andE_(C): El (%) in C direction.

Example 2

The rest of the pieces which were obtained by dividing the ingot of 50kg into two pieces in Example 1 was heated up to a temperature of 1170°C., and then hot rolled into a sheet bar having a thickness of 30 mm anda width of 150 mm. Thereafter, this sheet bar was subjected to forgingand made into a bar of 35 mm×35 mm, annealing at a temperature of 1040°C., then machined into a thermal fatigue test specimen having thedimensions illustrated in FIG. 1, and then used in a thermal fatiguetest as described below.

<Thermal Fatigue Test>

In a thermal fatigue test, a thermal fatigue life was observed byrepeatedly heating and cooling the test specimen between thetemperatures of 100° C. and 850° C. at a restraint ratio of 0.30. Here,a heating rate and a cooling rate were both 10° C./sec, a holding timeat a temperature of 100° C. was 2 minutes and a holding time at atemperature of 850° C. was 5 minutes. The thermal fatigue liferepresents as the number of cycles at which the stress first started tocontinuously decrease from that in the previous cycle. The stress wasderived by calculated as the quotient of the load detected at 100° C.divided by the cross section area of the soaked parallel portion of atest specimen indicated in FIG. 1.

The results of the continuous oxidation test in air, the continuousoxidation test in water vapor atmosphere, the high-temperature fatiguetest and the tensile test at room temperature in Example 1 and those ofthe thermal fatigue test in Example 2 are summarized in Table 2. AsTable 2 indicates, it is clear that any of the steel of the example ofthe present invention which is within the range of the present inventionhas heat resistance (oxidation resistance, a thermal fatigue propertyand a high-temperature fatigue property) equivalent to or better thanthat of SUS444 and excellent formability in terms of a mean elongationin the three directions (L, C and D direction) at room temperature of36% or more, which means it has been confirmed that the steel satisfiesthe object of the present invention. In contrast, the steel of thecomparative example which is out of the range according to the presentinvention is poor in either of oxidation resistance, thermal fatigueresistance, a high-temperature fatigue property or formability, whichmeans it has been confirmed that the steel does not satisfy the objectof the present invention.

INDUSTRIAL APPLICABILITY

The steel according to the present invention can be ideally used notonly for the parts of an exhaust system of an automobile but also theparts of an exhaust system of a thermal electric power system and theparts of a solid-oxide fuel cell for which similar properties as that ofthe parts of an exhaust system of an automobile are required.

TABLE 1 Sample No. C Si Mn Al P S Cr Cu Nb 1 0.008 0.84 0.25 0.51 0.0310.002 12.8 1.32 0.48 2 0.007 0.76 0.28 0.40 0.029 0.003 14.5 1.45 0.46 30.008 0.91 0.31 0.67 0.030 0.003 15.1 1.48 0.49 4 0.009 0.69 0.29 0.380.028 0.003 13.4 1.23 0.47 5 0.006 0.54 0.54 0.31 0.027 0.004 15.2 1.370.44 6 0.007 0.89 0.48 0.43 0.028 0.003 14.3 1.54 0.49 7 0.009 0.76 0.240.37 0.029 0.003 14.9 1.19 0.46 8 0.007 0.80 0.73 0.45 0.025 0.003 13.71.67 0.45 9 0.007 0.73 0.81 0.4 0.026 0.004 15.5 1.59 0.45 10 0.008 0.680.89 0.32 0.026 0.002 12.9 1.24 0.43 11 0.007 0.94 0.39 0.52 0.028 0.00213.5 1.55 0.45 12 0.006 1.34 0.5 0.37 0.024 0.003 14.0 1.15 0.48 130.007 0.69 0.44 1.49 0.025 0.002 12.7 1.46 0.47 14 0.008 0.92 0.78 0.130.027 0.003 15.6 1.29 0.48 15 0.009 0.47 0.63 0.68 0.029 0.002 14.4 1.510.46 16 0.008 0.81 0.35 0.54 0.026 0.003 13.9 0.53 0.44 17 0.008 0.520.21 0.39 0.030 0.004 17.1 1.46 0.48 18 0.007 0.76 0.84 0.43 0.027 0.0039.7 1.64 0.43 19 0.007 0.87 0.33 0.03 0.029 0.002 14.8 0.02 0.44 200.008 0.31 0.42 0.02 0.031 0.003 18.7 0.02 0.52 21 0.008 0.32 0.05 0.010.028 0.002 17.0 1.93 0.33 22 0.009 0.46 0.54 0.00 0.029 0.003 18.9 1.360.35 23 0.006 0.22 0.05 0.05 0.005 0.005 18.8 1.65 0.42 Sample mass %No. Ti Mo W N Others Si-Al Note 1 0.008 0.02 0.02 0.008 0.33 Example 20.007 0.03 0.01 0.009 0.36 Example 3 0.009 0.02 0.02 0.007 0.24 Example4 0.007 0.02 0.03 0.008 0.31 Example 5 0.006 0.01 0.02 0.008 0.23Example 6 0.008 0.01 0.03 0.007 0.46 Example 7 0.009 0.03 0.02 0.0060.39 Example 8 0.007 0.02 0.01 0.007 V: 0.21 0.35 Example 9 0.008 0.020.01 0.007 B: 0.0015 0.33 Example 10 0.008 0.01 0.02 0.008 Co: 0.09 0.36Example 11 0.007 0.01 0.02 0.007 Ni: 0.34 0.42 Example 12 0.006 0.010.03 0.006 0.97 Comparative Example 13 0.009 0.02 0.02 0.007 −0.8Comparative Example 14 0.007 0.02 0.02 0.008 0.79 Comparative Example 150.008 0.03 0.02 0.006 −0.21 Comparative Example 16 0.007 0.02 0.01 0.0080.27 Comparative Example 17 0.003 0.02 0.02 0.008 V: 0.06 0.13Comparative Example 18 0.008 0.01 0.03 0.007 0.33 Comparative Example 190.030 0.01 0.01 0.008 0.84 Comparative Example*1 20 0.003 1.87 0.020.008 0.291 Comparative Example*2 21 0.002 0.01 0.02 0.01 Ni: 0.10, 0.31Comparative V: 0.10 Example*3 22 0.080 0.01 0.02 0.007 Ni: 0.10, 0.458Comparative V: 0.03, Example*4 B: 0.0030 23 0.090 0.02 0.02 0.006 Ni:0.15 0.168 Comparative Example*5 Underline indicates the value out ofthe range according to the present invention. *1: Type429 *2: SUS444 *3:Example 3 of Patent literature 2 *4: Example 3 of Patent literature 3*5: Example 5 of Patent literature 4

TABLE 2 High- Temper- Weight ature gain by Mean Fatigue Weight Thermalwater Elongation Life at gain by Fatigue vapor in Three 850° C. Sampleoxidation Life oxidation Directions (×l 0⁻⁵ No. (g/m²) (cycle) (g/m²)(%) cycles) Note 1 18 1330 34 36 30 Example 2 17 1340 34 36 33 Example 316 1350 33 36 26 Example 4 17 1370 34 37 29 Example 5 16 1340 35 37 27Example 6 16 1310 33 36 32 Example 7 16 1370 34 37 31 Example 8 17 136034 36 36 Example 9 16 1340 34 36 27 Example 10 18 1300 34 37 29 Example11 17 1410 33 36 33 Example 12 17 1280 39 32 30 Comparative Example 1318 1380 71 31 9 Comparative Example 14 55 1400 40 37 15 ComparativeExample 15 17 1290 77 36 11 Comparative Example 16 15 910 32 38 26Comparative Example 17 13 1440 29 34 31 Comparative Example 18 521300 >100 38 14 Comparative Example 19 45 630 >100 34 8 ComparativeExample*1 20 19 1250 37 31 24 Comparative Example*2 21 >100 1650 >100 3115 Comparative Example*3 22 >100 1380 >100 35 10 Comparative Example*423 >100 1540 >100 34 12 Comparative Example*5 Underline indicates thevalue out of the range according to the present invention. *1: Type429*2: SUS444 *3: Example 3 of Patent literature 2 *4: Example 3 of Patentliterature 3 *5: Example 5 of Patent literature 4

1. Ferritic stainless steel having a chemical composition containing, bymass %, C: 0.015% or less, Si: 0.4% or more and 1.0% or less, Mn: 1.0%or less, P: 0.040% or less, S: 0.010% or less, Cr: 12% or more and lessthan 16%, N: 0.015% or less, Nb: 0.3% or more and 0.65% or less, Ti:0.15% or less, Mo: 0.1% or less, W: 0.1% or less, Cu: 1.0% or more and2.5% or less and Al: 0.2% or more and 1.0% or less, while therelationship Si≧Al is satisfied, and the balance being Fe and inevitableimpurities.
 2. Ferritic stainless steel having a chemical compositionfurther containing one, two or more chemical elements selected fromamong, by mass %, B: 0.003% or less, REM: 0.08% or less, Zr: 0.5% orless, V: 0.5% or less, Co: 0.5% or less and Ni: 0.5% or less.